1. Field of the Invention
The present invention relates to a locking device for locking a steering shaft so as to keep the shaft from turning while the car is not used, and to a steering apparatus provided with the locking device.
2. Description of Related Art
For preventing theft during parking, cars are provided with a locking device for locking a steering shaft which is used for transmitting rotation of a steering wheel to a steering mechanism.
FIG. 1 is a sectional view illustrating an essential part of a conventional locking device for a steering shaft. As disclosed in Japanese Patent Application Laid-Open No. 57-191149 (1982), for example, a conventional locking device has a key cylinder 109 of an engine key which is fastened to an upper portion of an upper shaft housing 107 for supporting a steering shaft 101 connected to a steering wheel 100. The key cylinder 109 is connected with a lock key 108 which slides into the upper shaft housing 107 by an operation of the key cylinder 109. A key way 110, which engages with the lock key 108 when the lock key 108 sides into the upper shaft housing 107, is formed on a collar 111 fastened on the peripheral surface of the steering shaft 101. The steering shaft 101 is locked and kept from turning when the lock key 108 and the key way 110 engage with each other.
Cars are also provided with an impact absorbing mechanism for reducing impact of collision or the like, which has caused an external force, when an excessively large external force is applied to the steering wheel 100 in the axial direction of the steering shaft 101 at the time of the collision.
A generally used impact absorbing mechanism is constituted of a hollow upper shaft 102 which has an upper end connected with the steering wheel 100 and an open lower end; a lower shaft 103 which has an upper end fitted in the lower end of the upper shaft 102 with relative turn thereof with regard to the upper shaft 102 being prohibited and movement thereof in the axial direction being allowed; a stopper 105, which is made of synthetic resin filled in a groove extending over the circumference of a portion of the lower shaft 103 to be fitted in the upper shaft 102 and a through bore formed on the upper shaft 102 in correspondence with the groove, for restraining the upper shaft 102 and the lower shaft 103 in the axial direction; the upper shaft housing 107 for supporting the upper shaft 102 with rotation thereof being allowed and relative movement thereof in the axial direction with regard to the upper shaft housing 107 being prohibited; a lower shaft housing 106, which supports the lower shaft 103 so as to be rotatable, having an upper end fitted in the lower end of the upper shaft housing 107; and a cylindrical impact energy absorber 104 interposed between a portion of the upper shaft housing 107 to be fitted with the lower shaft housing 106 and a portion of the lower shaft housing 106 to be fitted with the upper shaft housing 107.
With such a structure, when an excessively large external force is applied to the steering wheel 100 in the axial direction of the steering shaft 101 at the time of collision or the like, the stopper 105 is sheared at the interface between the upper shaft 102 and the lower shaft 103, so that relative movement of the upper shaft 102 in the axial direction with regard to the lower shaft 103 is allowed. In this state, the external force applied to the steering wheel 100 is transmitted to the impact energy absorber 104 via the upper shaft 102, a snap ring 112 and the upper shaft housing 107. As the impact energy absorber 104 absorbs energy of the external force, the upper shaft 102 and the upper shaft housing 107 move downward with regard to the lower shaft housing 106 within a predetermined range of stroke length, so as to absorb the impact of the collision which has caused the external force.
The locking device and the impact absorbing mechanism mentioned above can be employed for a vehicle having an electric power steering apparatus. Some electric power steering apparatuses are provided with a steering assist motor, wherein a driven gear of a reduction gear mechanism, which is driven by rotation of the motor, is fastened at the middle of a lower shaft of a steering shaft.
In a vehicle provided with an electric power steering apparatus in which a driven gear is fastened at the middle of a lower shaft of a steering shaft as described above, it is more difficult to ensure a stroke length of the impact absorbing mechanism, in comparison with a vehicle which does not have a steering assist motor or a reduction gear mechanism at the middle of the steering shaft. Consequently, a conventional vehicle having an electric power steering apparatus thus constructed can not meet a recent demand for an increase in stroke length to further enhance the safety at the time of collision.
Moreover, there is another demand for ensured strength of the key way high enough to prevent a plan for breaking the key way to steer the vehicle by applying large torque to the steering wheel repeatedly to make the lock key collide with a side face of the key way repeatedly.